If I can unlock my Verizon iPhone so I can use it on a local carrier's network while I'm on vacation in Europe, why can't I pop in a SIM card to that same unlocked phone, and use it on AT&T or T-Mobile when I get home?

In this edition of Ask Maggie, I explain why the term "unlocked" doesn't really mean "unlocked" when it comes to the iPhone 4S.

Why a Verizon Wireless unlocked iPhone 4S won't work on AT&T

Dear Maggie, I have a Verizon Wireless iPhone 4S. But I think the service is too expensive. I'd like to get a prepaid SIM card and use it on AT&T or T-mobile. I unlocked my iPhone 4S a few months ago when I traveled to Europe. And I used a SIM card from a local carrier while I was there. But when I came home to see if I could get a prepaid SIM from AT&T or T-Mobile, I was told by sales associates at Verizon and AT&T that even though my iPhone 4S is unlocked, I can't put a U.S. carrier's SIM card in it. Is this true?

That doesn't make sense to me! When I am abroad, my unlocked iPhone 4S will work on any other GSM carrier, so why won't it work like that on a GSM carrier in the U.S.?

If I cancel my contract and pay the cancellation fee, I should be able to use my phone on other carriers. Please tell me I'm not stuck with Verizon for as long as I have this iPhone!

Thanks, JD

Dear JD, Here's a short answer to your question. Unfortunately, the iPhone 4S that you bought from Verizon only works on Verizon's network here in the U.S.

Apple

I completely agree with you that this makes absolutely no sense given the fact that hardware components in an iPhone 4S from Verizon are exactly the same as the components in an iPhone 4S from AT&T and the same as the unlocked iPhone 4S sold throughout much of the world.

This of course is different from most other device launches, where carriers build different devices for different markets and even different carriers, depending on whether they are a CDMA carrier, like Verizon and Sprint are in the U.S. or whether they're a GSM carrier, like AT&T, T-Mobile, and most other operators in the world.

In theory, this should mean that a so-called "unlocked" iPhone 4S from any of the three major wireless carriers in the U.S. or the unlocked version available from Apple should work on any GSM network and any CDMA network. (Remember it has components that can handle both types of networks.)

But the reality is that the iPhone 4S is never really "unlocked." The carriers, which sell these phones directly to subscribers, require that the devices remain locked to their networks. This means that it's very difficult, if not nearly impossible, to use an iPhone 4S from Verizon on any other carrier's network in the U.S. And iPhones sold for other carrier networks in the U.S. will not operate on Verizon's network.

The Verizon and Sprint versions of the iPhone 4S are programmed to predominantly operate on CDMA networks. They can be "unlocked" for international use, which means that certain SIM cards can put in them to get access to foreign carrier's network. But if you try putting in a SIM card from AT&T or T-Mobile to an unlocked Verizon or Sprint iPhone 4S, you'll get an error message.

Marc Weber Tobias, a contributor to Forbes, wrote back in December that he tried this with a Verizon iPhone he purchased at full price from Best Buy, but was meant for the Verizon network. He explained how he had to jump through a series of hoops just to get his device unlocked from Verizon. He used his unlocked iPhone 4S just fine in Europe, but when he tried to insert a prepaid SIM card from a carrier that uses AT&T and T-Mobile's network, he got a message saying he was using an unauthorized SIM. But then he popped in his European carrier SIM card and his phone connected via the foreign carrier to AT&T's network.

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While unlocked iPhones bought from Apple or from AT&T can accept any SIM card for access to any GSM carrier, including T-Mobile in the U.S., these phones can't be activated on Verizon or Sprint.

It's true that Verizon and Sprint use a different fundamental network technology from what AT&T and T-Mobile use. But keep in mind that the iPhone 4S is exactly the same in terms of hardware regardless of carrier. This means it has a CDMA radio as well as a GSM radio.

But on the "unlocked" iPhone, which can be bought at full price from Apple, and on an unlocked AT&T iPhone, the CDMA portion of the phone is disabled and unusable.

Unfortunately, I haven't heard of a way for consumers to change this. I'm sure some people have tried. But even if you're successful in breaking the lock on the device, there's a good chance that some of the services may not function properly. This means that you may not be able to send or receive text messages or visual voice mail may be broken.

At any rate, it's not an easy hack and it may render your device useless. To sum up, I'm sad to say that if you buy an iPhone 4S from Verizon, you're pretty much stuck using it on Verizon. This may be why the resale value of a used iPhone 4S from Verizon or Sprint is still less than for an AT&T version of the same device.

What about using my Sprint iPhone 4S on Virgin Mobile? They use the same network.

Dear Maggie, I just discovered your column. Thanks for all the informative stories. Here's my question. I am a Sprint customer with an iPhone 4S. I've noticed that Virgin Mobile, which uses Sprint's network, now offers the iPhone. The price of the Virgin Mobile service is $55, which is a lot less than what I pay now. Could I cancel my service with Sprint, pay the early termination fee and take my iPhone 4S? After all, don't they use the same network as Sprint?

I'd rather not buy another iPhone 4S.

Thanks, Ray

Dear Ray, This is a great follow-up question to the question I just answered from J.D. As I described in the previous answer, the short answer is "no."

Josh Miller/CNET

I have to admit as a consumer I find this fact incredibly annoying. You are absolutely correct about the relationship between Virgin Mobile and Sprint. Virgin Mobile is a prepaid brand that uses the Sprint network to deliver service. It's even owned by Sprint Nextel.

The only major difference in terms of the service is the fact that Virgin Mobile only uses Sprint's network. It doesn't have access to any of Sprint's roaming partners, so it's footprint is slightly more limited that Sprint's coverage. And the Virgin Mobile iPhone cannot roam overseas, since Virgin doesn't have any roaming partners.

Other than that, Virgin Mobile uses the exact same radio frequency and the exact same network to delivers services for its customers as Sprint uses to deliver to its subscribers.

So why can't a Sprint iPhone 4S operate on Virgin Mobile's network? The short answer to the question is because Sprint and Virgin Mobile would rather lock customers into those services rather than allow customers to decide for themselves if they want a prepaid service or a subscription service.

As I explained in the previous answer, even though every version of the iPhone 4S sold in the U.S. market uses the same exact hardware, individual carriers lock the phone to their own specific services. And that means that Virgin Mobile, even though it's owned by Sprint and uses the same network as Sprint, has its own lock for the iPhone.

This prevents consumers from doing exactly what you want to do, which is to take a device that you have paid for and use it on another network, that offers the exact same piece of hardware.

One easy explanation is that Virgin Mobile and Sprint want to keep customers locked into their services. But a Virgin Mobile spokeswoman said there is more to it. She said that none of the phones that operate on Sprint's network can be used on Virgin Mobile. She said that this is largely due to the different back-end systems running each brand of service. After all, Virgin Mobile is a separate brand. It's also not a brand that was developed by Sprint. It was acquired by Sprint a few years ago.

Still, the "locks" the carriers impose on the iPhone 4S seem ridiculous to me. An iPhone 4S from AT&T, Verizon, Sprint or even Virgin Mobile should work on any network that supports and sells the same iPhone 4S. As I said before, the hardware is exactly the same. The carriers know how these devices will behave on their networks. So there is no worry that bringing this device from another carrier will adversely affect the network. The only reason carriers block other iPhone 4S's from being used on other their networks is because they want to make it more difficult for consumers to quit their service and go someplace else.

Carriers are using these software locks to force customers to stick with their service. They don't want subscribers to be able to bring their own devices or re-use the same exact device from another carrier on their own networks.

What this means for consumers is that if they want to switch carriers they have to repurchase the exact same piece of hardware to use it on another network. Like the early termination fees that lock customers to a particular service, this type of device "locking" is nothing more than a way to quash consumer choice.

Unfortunately, the situation isn't likely to get any better when the new iPhone 5 is released. For one, carriers will likely continue to put software locks on their devices. But the new iPhone is also likely to support 4G LTE. And even though Verizon, Sprint, and AT&T are each using the same 4G LTE technology to build their next generation networks, the band plans and frequencies used by each carrier are different. And this means that one device may not have the componentry to access all 4G LTE networks.

But that's a completely different topic, which I have covered in previous Ask Maggie columns.

I'm sorry I had to deliver such bad news. I hope I didn't ruin your weekend.

Ask Maggie is an advice column that answers readers' wireless and broadband questions. The column now appears twice a week on CNET offering readers a double dosage of Ask Maggie's advice. If you have a question, I'd love to hear from you. Please send me an e-mail at maggie dot reardon at cbs dot com. And please put "Ask Maggie" in the subject header. You can also follow me on Facebook on my Ask Maggie page.